Conventional solenoids of this type include the one shown in FIGS. 4 and 5, for example.
A solenoid 50 is formed by integrating a molded coil 51 and a solenoid body 52 into one piece.
The molded coil 51 is wound with a coil 53. At the center of the coil 53, a hole 54 hollow toward the solenoid body 52 is formed.
Opposite to the solenoid body 52, the molded coil 51 is provided with an annular plate 55 adjacent to the coil 53. At the front of the plate 55, a molded portion 56 forming the bottom of the hole 54 is provided. In the molded portion 56, a coil wire drawn out from the coil 53 is connected to a lead wire 57.
On the other hand, the solenoid body 52 is provided, at an axially central portion, with a column portion 58 inserted in the hole 54 of the molded coil 51, and a case 59 covering the outer perimeter of the molded coil 51 at a distance from the column portion 58 with a portion in which the molded coil 51 is disposed as an annular space.
The column portion 58 of the solenoid body 52 is provided with a magnetic attraction portion 60 formed integrally with the case 59 by one member, a plunger 61 magnetically attracted to the magnetic attraction portion by passing current through the coil 53, a rod 62 connected to the plunger 61, a bearing 63 supporting the rod 62, a sleeve 64 covering the distal end of the column portion 58, and a cylinder 65 made from a non-magnetic material forming a magnetic interruption portion between the sleeve 64 and the magnetic attraction portion 60.
A plug 66 is disposed on the inside-diameter side of the magnetic attraction portion 60. A second bearing 68 is fitted in the inside-diameter side of the plug 66.
The magnetic attraction portion 60 extends from the inside-diameter end of an annular radial portion 67 toward the molded coil 51. The case 59 also extends from the outside-diameter end of the annular radial portion 67 toward the molded coil 51.
In the solenoid 50 of the above configuration, by supplying current to the coil 53 through the lead wire 57, as shown in FIG. 5, a magnetic circuit L through the magnetic attraction portion 60, the case 59, the plate 55, the sleeve 64, and the plunger 61, and back to the magnetic attraction portion 60 is formed, and the plunger 61 is magnetically attracted to the magnetic attraction portion 60.
When no current is supplied to the coil 53, under a biasing force by a spring or the like not shown, the plunger 61 is located away from the magnetic attraction portion 60 as shown in FIG. 4.
Of a solenoid as described above (Hereinafter, it is referred to as “Conventional Art 1.” See Patent Document 1, for example), attraction force and flat characteristics in a necessary stroke are required within required solenoid body dimensions (size).
However, in Conventional Art 1, as shown in FIG. 5, the extent of a magnetic flux transfer portion A between the plunger 61 and the sleeve 64 is small near an attraction end state of the plunger 61 (a state where the plunger 61 is in proximity to the plug 66), and magnetic flux cannot be transferred adequately. This results in a problem that attraction force is reduced, and flat characteristics cannot be obtained in the entire range of a necessary stroke.
A solenoid in which, in order to reduce variations in magnetic attraction force due to stroke change without increasing body dimensions, a hole in which a portion of a second stator core containing a movable core is inserted is provided in a yoke to increase the axial length of a magnetic transmission portion is known (Hereinafter, it is referred to as “Conventional Art 2.” See Patent Document 2, for example).
However, in Conventional Art 2, it is necessary to increase the axial lengths of two members, the movable core and the second stator core, as a set, increasing the material cost and the production cost. Further, the entire movable core is lengthened, so that there is a problem that the weight of a movable portion is increased, worsening responsivity.